Support the following:
- Shifts. ARM64 machine instructions only use lowest 6 bits of the
  shift (i.e. mod 64). Use conditional selection instruction to
  ensure Go semantics.
- Zero/Move. Alignment is ensured.
- Hmul, Avg64u, Sqrt.
- reserve R18 (platform register in ARM64 ABI) and R29 (frame pointer
  in ARM64 ABI).

Everything compiles, all.bash passed (with non-SSA test disabled).

Change-Id: Ia8ed58dae5cbc001946f0b889357b258655078b1
Reviewed-on: https://go-review.googlesource.com/25290
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>